Field of Invention
Exemplary embodiments of the present disclosure relate to a memory system and an operating method thereof.
Description of Related Arts
In NAND flash storage systems, the read reference voltages are used to divide the possible threshold voltage range of NAND flash memory cells to several windows. A logical value is designated to each of the windows. The logical value of a cell is determined by the voltage window which the cell's threshold voltage is located in. For example, for a single-level cell (SLC), one read reference voltage is needed. When the threshold voltage of a cell is less than the specified read reference voltage, the logical value of the cell sets to “1”; otherwise, the cell's logical value sets to “0”. For a multi-level cell (MLC), three read reference voltages are used. For a triple level cell (TLC) device, there are seven read reference voltages.
The threshold voltage of a cell can vary from its intended value for various reasons, such as program and read operation of neighboring cells, the number of program/erase cycles, and the retention time. Therefore, it is possible that the threshold voltage of a cell with an intended value shifts into another voltage window with a different logical value. In this case, when the cell is read, a wrong logical value is given and this error will be counted in the raw bit error rate (RBER) measurement. An optimal read reference voltage can achieve the minimal RBER.
Currently, there are two types of algorithm for the error-correction control solution, the hard-decoding algorithm and the soft-decoding algorithm. For the hard-decoding algorithm, the hard information which indicates that the input information is “0” or “1” is used. However, the soft information is used for the soft-decoding algorithm, which not only tells the decoder if the input information is “0” or “1”, but also provides the decoder how likely it is “0” of “1”. Usually if soft information is provided, a soft-decoding algorithm provides a better error correction performance than a hard-decoding algorithm. In NAND flash storage systems, an error-correction control solution will try the hard decoding first. When the hard decoding fails, the soft-decoding algorithm will be used.
Thus, there remains a need for a memory system and operating method thereof for read threshold estimation and adjustment.